Flash memory module for realizing high reliability

ABSTRACT

A flash memory module may include a plurality of flash memory chips. The memory chips may include one or more blocks. Each block may be a unit of erasing data. A flash controller may be coupled to the plurality of flash memory chips. The flash controller may program data to block and erase data from a block. The flash controller may manage a recent programming time for each of the plurality of blocks. The flash controller may erase data stored in a block for which an elapsed programming time is larger than a first value.

TECHNICAL FIELD

The present invention relates to storage control for programming data toan electrically rewritable nonvolatile semiconductor memory.

BACKGROUND ART

In a nonvolatile semiconductor memory, an electron is sent through anoxide insulation layer to a floating gate in a process for storing(programming) data to an internal cell. Furthermore, an electron isremoved from the FG (Floating Gate) through the oxide insulation layerin a process for erasing (erase) the stored data.

A nonvolatile semiconductor memory comprising this kind of data storagemechanism puts a load on the oxide insulation layer at each programmingand erase process. Repeating this load creates multiple electron trapsand positive hole traps inside the oxide insulation layer.

The nonvolatile semiconductor memory measures a threshold voltage, whichfluctuates in accordance with the charge (number of electrons) insidethe FG, to determine the recorded data. For this reason, when electrontraps and positive hole traps are formed inside the oxide insulationlayer as mentioned hereinabove, the electrons in these electron trapsand positive hole traps give rise to residual voltage, and this voltagecauses the FG threshold voltage to fluctuate, making it impossible toaccurately determine the recorded data.

The threshold voltage fluctuations resulting from electron traps andpositive hole traps like these are known to be a function of the controlinterval and the cumulative number of times that programming and erasingare executed. Furthermore, changes in the data storage timecharacteristics (hereinafter, referred to as retention characteristics)in accordance with the fluctuation of the threshold voltage is alsoknown.

Since a charge that has accumulated in the FG will change in accordancewith the passage of time, the data storage period of a nonvolatilesemiconductor memory is limited. In addition, the data storage period isfurther shortened by the fluctuation of the threshold voltage resultingfrom the above-mentioned electron traps and positive hole traps.

A semiconductor storage apparatus that uses a nonvolatile semiconductormemory as the storage medium (hereinafter, referred to as the storageapparatus) must satisfy the data storage period criteria stipulated inthe apparatus specifications. The storage apparatus predicts thedeterioration of the oxide insulation layer based on the number of timesthat data is programmed and erased, and in a case where it is predictedthat the data storage period of a specific area of on-board nonvolatilesemiconductor memory will not meet the criteria, makes this storage areaunusable.

To ensure that operation can continue even when an unusable area such asthis has occurred, the storage apparatus is generally equipped withspare storage areas in excess of the total amount of stored data of theapparatus, and exercises control such that this spare storage area isallocated in place of the above-mentioned area that has been determinedto be unusable so as to constantly maintain a fixed storage area.However, when this allocated spare area is exhausted, the storageapparatus loses its ability to record new data and shuts down.

Therefore, to realize a long-life storage apparatus, it is necessary tohold the fluctuation of the threshold voltage in accordance with theelectron traps and positive hole traps in check, and to prevent the datastorage period of a storage area from being shortened.

For example, Patent Literature 1 discloses control for lessening theeffects of these electron traps and positive hole traps. In the PatentLiterature 1, there is disclosed a storage apparatus, which makes anarea whose retention characteristics have worsened unusable for a fixedperiod, reduces the electron traps and position hole traps, and reusesthe storage area (Patent Literature 1).

CITATION LIST Patent Literature

PTL 1: US Patent Application No. 2010/0165689 A1

SUMMARY OF INVENTION Technical Problem

For example, a nonvolatile semiconductor memory (typically a flashmemory) is configured from multiple blocks, and this block is thesmallest erase unit. Then, as described hereinabove, because electrontraps and positive hole traps are formed in the oxide insulation layerin accordance with repeated erasing and programming, an upper limit isprovided on the number of erases for each block.

Generally speaking, the semiconductor memory is not capable ofoverwriting, and a programmed area must be erased before this area canbe used for programming once again. Therefore, erasing and programmingare always carried out as a set when programming new data to a block inwhich data is programmed. For this reason, the storage apparatus managesthe number of times that erasing has been carried out for a block(hereinafter, referred to as number of block erases) by block, treats ablock, which has exceeded the upper limit for number of block erases, ashaving exceeded the data storage period (specifically, the period oftime that data can be held and stored reliably), designates this blockas being unusable, and allocates a spare block as a substitute.

In a case where erasing and programming have concentrated on a specificgroup of blocks, there is the likelihood of generating large numbers ofblocks for which the number of block erases reaches the upper limit in ashort period of time, rendering the blocks unusable. Then, since spareblocks must be allocated to these blocks, the spare blocks will berapidly exhausted. That is, concentrating access on a specific blockgroup speeds up the exhaustion of spare blocks, shortening the life ofthe storage apparatus.

The storage apparatus disperses block programming processes to preventaccess and programming from concentrating on a specific block group likethis, and carries out a process for equalizing the number of erases perblock (hereinafter, this processing will be referred to as wearleveling). Wear leveling is a process for preferentially programmingdata to a block with a lower number of erases than an average value ofthe number of erases of blocks mounted in the storage apparatus.

For this reason, in a case where there is a block (hereinafter, will bereferred to as the target block) for which the average value of thenumber of block erases of the blocks in the storage apparatus is X(where X is an integer equal to or larger than 1), and the number ofblock erases within a block in the storage apparatus is Y (where Y is aninteger that satisfies the condition Y<X), this target block will beused preferentially. For this reason, programming and erasing are likelyto be repeated in a short period of time with respect to this targetblock.

In general, the data storage period increases the shorter the timebetween erasing a block and programming data to this block (hereinafter,will be referred to as the EP interval).

For this reason, even though the number of block erases of a certainblock is small, the repeated preferential use of this block shortens thetime from programming to erasing with respect to this block(hereinafter, will be referred to as the PE interval). As a result,there is the likelihood of large numbers of blocks for which the datastorage period has elapsed being generated in a short period of timeeven though the number of block erases of this block has not reached theupper limit value. In accordance with this, there is an increasedlikelihood of the life of this storage apparatus being shortened.

That is, there are cases in which it will be impossible to maintain thereliability of the storage apparatus simply by counting the number ofblock erases and performing wear leveling.

Therefore, an object of the present invention is to realize highreliability and long life in a semiconductor storage apparatus that usesnonvolatile semiconductor memory as the storage medium.

Solution to Problem

A semiconductor storage apparatus stores management informationcomprising, for each block of a nonvolatile semiconductor memory,information denoting at least one of a recent programming time, which isa time at which data is recently programmed to a block, and a recenterase time, which is a time at which an erase process is recentlyperformed with respect to a block. The semiconductor storage apparatus(b1) controls a timing at which data is programmed to a block based onat least one of the recent programming time or the recent erase time ofthis block, and/or (b2) controls a timing at which an erase process isperformed with respect to a block based on the recent programming timeof this block.

For example, the semiconductor storage apparatus may control the timeinterval of each programming and erase process, which are normalprocesses of the semiconductor storage apparatus, while carrying out theequalization of the number of erases using wear leveling to ensure aminimal time.

Furthermore, for example, the semiconductor storage apparatus mayexercise control so as to ensure the time intervals of programming andan erase process with respect to each block for as long as possiblerelative to a request from a higher-level apparatus (for example, a hostcomputer).

Also, for example, when a block with poor data retention characteristicsis detected, the semiconductor storage apparatus may implement controlso as to lower the utilization frequency of this block. A reduction inelectron traps and positive hole traps can be expected in accordancewith this, and therefore retention characteristics can be expected torecover.

Advantageous Effects of Invention

According to the present invention, it is possible to heighten thereliability and extend the service life of a semiconductor storageapparatus.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of the configuration of a computer system 1comprising an FM module 100 related to Example 1.

FIG. 2 shows an example of the configuration of a memory chip 120.

FIG. 3 shows an example of the internal configuration of a block 202.

FIG. 4 shows an example of data stored in a page 301.

FIG. 5 shows an example of an address conversion table 500.

FIG. 6 shows an example of a block management table 600.

FIG. 7 shows an example of a wear leveling management queue 700.

FIG. 8 shows an example of programs stored in RAM 114.

FIG. 9 shows an overview of erase process control.

FIG. 10 shows an overview of programming process control.

FIG. 11 shows an example of the flow of a programming process.

FIG. 12 shows an example of the flow of processing in Step S1102.

FIG. 13 shows an example of the flow of read processing.

FIG. 14 shows an example of the flow of reclamation processing.

FIG. 15 shows an example of a PE interval management queue 1500.

FIG. 16 is an outline diagram showing a block utilization cycle inExample 2.

FIG. 17 shows an example of the flow of invalid block creationprocessing.

FIG. 18 shows an example of the flow of erase processing.

FIG. 19 shows an example of the programs stored in RAM 114.

FIG. 20 shows an example of a table showing the relationship between thenumber of elapsed days and a failed bit threshold.

FIG. 21 shows an example of a management screen 2100.

FIG. 22 shows an example of the flow of erase processing in Example 3.

FIG. 23 shows an example of the flow of read processing in Example 3.

DESCRIPTION OF EMBODIMENTS

A number of examples of the present invention will be explained below.

Furthermore, in the following explanation, various types of informationmay be explained using the expression “xxx table”, but the variousinformation may also be expressed using a data structure other than atable. To show that the various information is not dependent on the datastructure, “xxx table” can be called “xxx information”.

Furthermore, in the following explanation, a number is used asidentification information for identifying an element, but another typeof identification information (for example, alphanumeric characters) maybe used either instead of or in addition to a number as theidentification information.

Also, in the following explanation, there may be cases where processingis explained having a “program” as the doer of the action, but since thestipulated processing is performed in accordance with a program beingexecuted by a processor (for example, a CPU (Central Processing Unit))while using a storage resource (for example, a memory) and/or acommunication interface device (for example, a communication port) asneeded, the processor may also be used as the doer of the processing. Aprocess, which is explained using the program as the doer of the action,may be regarded as a process performed by an apparatus comprising thisprocessor (for example, a storage control apparatus or a storageapparatus). Furthermore, the processor may comprise a hardware circuitthat carries out either part or all of the processing either instead ofor in addition to a microprocessor like the CPU. A computer program maybe installed in respective apparatuses from a program source. Theprogram source, for example, may be either a program delivery server ora computer readable storage medium.

Furthermore, the management computer may be configured using one or morecomputers. For example, in a case where either the management computerdisplays information or the management computer sends displayinformation to a remote computer, a single computer is the managementcomputer. Also, for example, in a case where the same functions as thoseof the management computer are realized using multiple computers, therelevant multiple computers (may include a display computer in a casewhere a display is made by the display computer) are the managementcomputer.

Furthermore, in the following explanation, in a case where anexplanation is given without making a distinction between elements ofthe same type, only the sign that is common to both elements within thereference sign will be used, and in a case where an explanation is givenby making a distinction between elements of the same type, the referencesign (a combination of the common sign and a specific sign) will beused.

EXAMPLE 1

FIG. 1 shows an example of the configuration of a computer system 1comprising a semiconductor storage apparatus 100 related to Example 1 ofthe present invention. Furthermore, because a semiconductor memory istypically a FM (Flash Memory), the semiconductor storage apparatus willbe called a “FM module” hereinbelow.

The computer system 1 comprises a host computer 10, a managementcomputer 11, and a semiconductor storage apparatus (will be called theFM (Flash Memory) module hereinbelow) 100.

The FM module 100 is coupled to the host computer 10 via a first FE-IF(Front End-Interface) 111, and is coupled to the management computer 11via a built-in second FE-IF 111. The communication protocol between theFM module 100 and the host computer 10 and the communication protocolbetween the FM module 100 and the management computer 11 may either bethe same protocol or a different protocol.

The host computer 10, for example, is a computer, a file server, or astorage apparatus to which large numbers of FM modules 100 are coupled,which serves as the core of a business system.

The host computer 10, for example, comprises hardware resources, such asa not-shown processor, memory, network interface, and local input/outputdevice. Furthermore, the host computer 10 comprises software resources,such as a device driver, an OS (Operating System), a management program,and an application program.

The processor of the host computer 10 can execute the various programs(software resources). For example, by the processor of the host computer10 executing an application program, the host computer 10 is able tocommunicate with the FM module 100 and issue a data read/write commandto the FM module 100. Furthermore, for example, by the host computer 10processor executing a management program, the host computer 10 is ableto acquire management information, such as the utilization status andprocessing status of the FM module 100. Furthermore, the host computer10 can specify the management unit and the control method of the FMmodule 100, and can change the control method.

The management computer 11 can manage the entire computer system 1. Themanagement computer 11, for example, comprises a not-shown input device,such as a keyboard or mouse, a processor, a display device, such as aliquid crystal display device, and a storage resource, such as a memory.Various programs are stored in this storage resource, and these programsare executed by the processor to realize various functions.

The FM module 100 comprises a FM controller 110, and multiple (forexample, 32) nonvolatile semiconductor memory (hereinafter, simplycalled memory) 120. Multiple RAID groups may be configured based on themultiple memory chips 120. A RAID group may be configured using two ormore memory chips 120. A logical volume may be configured based on aRAID group. The logical volume may be provided to the host computer 10,or may be a type of logical volume (for example, a pool volume, which isa logical volume that makes up a pool) that is not provided to the hostcomputer 10. The pool volume may be partitioned into two or more areas(hereinafter, actual areas). An actual area may be allocated to avirtual area, which makes up a virtual logical volume conforming to ThinProvisioning. At least one of the multiple pool volumes making up thepool may be a virtual logical volume to which is mapped a logical volumeof an external storage apparatus (not shown in the drawing) coupled tothe FM module 100.

The FM controller 110, in this example, is a controller that is builtinto the FM module 100, but may also be a computer, which exists outsideof the FM module 100. The FM controller 110 comprises a FE-IF (FrontEnd-Interface) 111, a switch 112 for transferring data back and forth, astorage resource (for example, a data buffer 113, and a RAM (RandomAccess Memory) 114), a processor 115, and a FM-IF (FlashMemory-Interface) 116.

The switch 112 is coupled to the processor 115, the RAM 114, the databuffer 113, the FE-IF 111 and the FM-IF 116, and transfers and routesdata in accordance with either an address or an ID.

The FE-IF 111 is coupled to various components of the FM controller 110,the host computer 10 and the management computer 11 via the switch 112.The FE-IF 111 receives a read/write command and a LBA (Logical BlockAddress), which specifies a target logical storage location, from thehost computer 10. Upon receiving a write command from the host computer10, the FE-IF 111 also receives write data from the host computer 10.Furthermore, the FE-IF 111 receives a control command and the like forthe FM module 100 from the host computer 10 and the management computer11, and controls the FM module 100 in accordance with this command. TheFE-IF 111 also notifies the host computer 10 and the management computer11 of the processing status, the utilization status and the currentsetting values of the FM module 100.

The data buffer 113 stores temporary data that is in the midst of a datatransfer process in accordance with the FM controller 110.

The RAM 114, specifically, is a volatile memory, such as a DRAM (DynamicRandom Access Memory). The RAM 114 stores the memory chip 120 managementinformation 1140, which is used by the FM module 100, and a transferlist, which includes transfer control information used by each DMA(Direct Memory Access). Furthermore, the RAM 114 includes either aportion or all of the functions of the data buffer 113 for storing data,and can also be used as a temporary storage destination for data.

The processor 115 is coupled to the respective components of the FMcontroller 110 via the switch 112, and controls the entire FM controller110 based on the programs and management information stored in the RAM114. Furthermore, the processor 115 monitors the entire FM controller110 by regularly acquiring information and an interrupt receptionfunction.

The FM-IF 116 is coupled to the memory chip 120 via multiple buses (forexample, 16). Multiple memory chips 120 (for example, two) are coupledto each bus. Each memory chip coupled to each bus is independentlycontrolled on the basis of a CE (Chip Enable) signal.

The FM-IF 116 performs processing in accordance with a read/writecommand specified from the processor 115. At this time, the FM-IF 116 isinstructed as to the command target in accordance with a physicaladdress (hereinafter, PBA (Physical Block Address)).

Upon receiving a PBA, the FM-IF 116 computes the required memory chip120 (blocks, pages) based on this PBA, and processes the read/writecommand by specifying a block and a page in the memory chip 120.

At read processing, the FM-IF 116 reads the data from the memory chip120 and transfers this data to the data buffer 113. Furthermore, atwrite processing, the FM-IF 116 calls the write data from the databuffer 113, and writes this data to the memory chip 120.

Furthermore, the FM-IF 116 comprises an ECC (Error Correcting Code)creation circuit, an ECC-based data loss detection circuit, and an ECCcorrection circuit. At write processing, the FM-IF 116 adds the FCC tothe data and carries out the write. Furthermore, at the time of a datacall, the FM-IF 116 checks the called data from the memory chip 120 bythe FCC-based data loss detection circuit. When data loss is detected,the FM-IF 116 performs data correction in accordance with the ECCcorrection circuit, and in order to notify the processor 115 of thenumber of correction bits, stores the number of correction bits in theRAM 114.

The above-described FE-IF 111, switch 112, data buffer 113, RAM 114,processor 115, and FM-IF 116 may be configured inside a singlesemiconductor device as an ASIC (Application Specific IntegratedCircuit) or FPGA (Field Programmable Gate Array), or the configurationmay be such that multiple individual ICs (Integrated Circuits) arecoupled to one another.

FIG. 2 shows an example of the configuration of the memory chip 120.

The memory chip 120 is configured from multiple (for example, 4,096)blocks 202. Each block 202 is configured using multiple pages. In thememory chip 120, erase processing is performed in units of blocks, anddata read/programming are performed in units of pages. The memory chip120, for example, is a NAND flash memory, which is capable of erasingdata in block units. The flash memory does not have to be the NAND type(for example, a NOR type can also be used).

The memory chip 120 comprises a register 201. The register 201 has astorage capacity of equal to or larger than page size (for example, 4KB+a spare area for adding an ECC).

The memory chip 120 performs processing in accordance with theinstructions of a read-write command from the FM-IF 116.

In a write process, the memory chip 120 first receives a write commandand write-destination information (for example, information denoting thewrite command target block, page number, and in-page program startlocation) from the FM-IF 116.

Next, the memory chip 120 stores the write data transferred from theFM-IF 116 in the register 201 in order from the in-page program startlocation (address). Thereafter, the register 201, upon receiving adata-transfer-complete command from the FM-IF 116, writes (programs) thedata stored in the register 201 to the specified memory chip 120 (page).

In a read process, the memory chip 120 first receives a read command andread-source information (for example, a read command target block, and apage number) from the FM-IF 116. Next, the memory chip 120 reads thedata stored in the page of the specified block 202, and stores this datain the register 201. Thereafter, the register 201 transfers the datastored in the register 201 to the FM-IF 116.

FIG. 3 shows an example of the internal configuration of a block 202.

The block 202 is divided into multiple (for example, 128) pages 301, anda data read and a data write can only be processed in page units.Furthermore, the order for writing to a page 301 inside the block 202 isestablished, and in the drawing, data is sequentially written in orderfrom page 1.

In the FM module 100, the host computer 10-specified LBA and the addressPBA (Physical Block Address), which specifies a physical storagelocation inside the FM module 100, are managed in accordance withdifferent address systems, and the corresponding relationship betweenthe LBA and the PBA is managed as table information (refer to FIG. 5).

In the FM module 100, it is not possible to overwrite a page to which awrite has been completed, and this page cannot be written to again untilan erase process has been carried out with respect to the block to whichthis page belongs. Therefore, when updating data, the page associatedwith the LBA is changed to a new page prior to updating, and data iswritten to this new page. The host computer 10 does not know that a newpage has been associated with the ILA like this. Furthermore, the pageassociated with the LBA becomes an invalid page prior to updating.

Furthermore, the PBA is an address system, which can uniquely specify anarbitrary page of the FM module 100, and arranges addressesconsecutively in page number order within a range that is at least oneblock worth of data size. For example, when one page worth of data sizeis incremented with respect to a PBA specifying a third page in acertain block, the PBA transitions to a fourth page.

FIG. 4 shows an example of data stored in a page 301.

The page 301 stores a fixed number of bits (for example, 4 KB) of data.In addition to data 401, an ECC (Error Correcting Code) 402, which wasappended by the FM-IF 116, is also stored in the page 301. The ECC isstored adjacent to the data 401 to be protected (hereinafter, will becalled the protected data). The data 1 401 and the ECC 402 added to thedata 1 401 are stored as a set so as to configure an ECC CW (ECCCodeWord).

Furthermore, a configuration in which four ECC CWs are stored in onepage is shown in FIG. 4, but the number of ECC CWs can be an arbitrarynumber in accordance with the page size and the performance of the ECC(the number of correctable bits). Furthermore, a data loss failure inthe above-described configuration is a phenomenon that occurs when thenumber of failed bits per single ECC CW exceeds the number ofcorrectable bits of the ECC that makes up this ECC CW.

The configuration of the FM module 100 and the storage configuration ofthe data of the FM module 100 have been explained hereinabove.

FIG. 5 shows an example of an address conversion table 500.

The address conversion table 500 is one of the pieces of managementinformation 1140 stored in the RAM 114 of the FM module 100.

The address conversion table 500 correspondingly manages a LBA 501 and aPBA 502 for each page (4 KB in the example of FIG. 5).

The LBA 501 denotes the first address of data stored in a single page.The PBA 502 denotes the address, which uniquely identifies a pageconfiguring the memory chip 120. Furthermore, in a case where a PBA 502has not been associated with an LBA 501, information denotingunallocated is stored in the LBA 501.

In this example, each time new data is written to a page correspondingan LBA 501, the FM controller 110 associates a new PBA 503 in place ofthe LBA 501 associated with this LBA 501. The FM controller 110associates a new PBA 502 with the LBA 501 specified from the hostcomputer 10, and registers this association in the address conversiontable 500. Furthermore, in FIG. 5, an example in which the LBA 501 andthe PBA 502 have been associated in units of page size is shown, but thepresent invention is not limited to this unit. For example, the LBA 501and the PBA 502 can also be associated in units of block size.

Next, management information for carrying out the time control, which isa characteristic feature of this example, will be explained.

FIG. 6 shows an example of a block management table 600.

The block management table 600 is one of the pieces of managementinformation stored in the RAM 114 of the FM module 100.

The block management table 600 correspondingly manages for each block (1MB in the example of FIG. 6) 202 a PBA group 601, a chip number 602, ablock number 603, the number of erases 604, a previous erase date/time605, a first page programming date/time 606, a last page programmingdate/time 607, a last programmed page 608, and the number of invalidpages 609.

The PBA group 601 denotes the first address (the first PBA) of the block202 (refer to FIG. 2).

The chip number 602 shows the number of the memory chip 120 comprisingthe block 202. In this example, as shown in FIG. 6, a chip number 602having a low address is associated in order starting from a PBA group601 with a low address.

The block number 603 shows the number of the block. In this example, ablock number 603 having a low address is associated in order startingfrom a PBA group 601 with a low address as shown in FIG. 6.

The number of erases 604 shows the number of times that erase processinghas been carried out for the block identified in accordance with theblock number 603. As used here, erase processing denotes erasing all thedata that exists in a block 202, and re-erasing data inside a block 202even when no data exists in the block 202. In this example, the numberof erases 604 is incremented by 1 each time erase processing is carriedout.

Previous erase date/time 605 denotes the date and time at which eraseprocessing was recently carried out with respect to the block identifiedin accordance with the block number 603. In this example, when eraseprocessing is carried out for a block, the date/time of this eraseprocessing is registered as the previous erase date/time 605.

The first page programming date/time 606 denotes the date and time atwhich data was programmed to the first page of the block identified inaccordance with the block number 603. As used here, “programming”denotes the writing of data to a block 202 (a page 301) from theregister 201. In this example, when programming data to the first page,the date/time of this programming is registered as the first pageprogramming date/time 606. Furthermore, a value denoting either “allpages erased” or “no write” is registered in the first page programmingdate/time 606 with respect to a block for which programming has not beencarried out to the first page. Hereinafter, writing data from the hostcomputer 10 to the FM module 100 will be called a “write”, and writingdata inside the FM module 100 will be called “programming”.

The last page programming date/time 607 shows the date and time at whichdata was programmed to the last page of the block 202 identified inaccordance with the block number 603. In this example, each time data isprogrammed to the last page, the date/time of this programming isregistered as the last page programming date/time 607. Furthermore, avalue denoting that writing to all the pages in the block is incompleteis registered as the last page programming date/time 607 with regard toa block 202 for which programming to the last page has not been carriedout.

The last programmed page 608 shows the number of the page for whichprogramming has been completed up to the current point in time in theblock identified in accordance with the block number 603. In thisexample, the next programming-destination page 301 is specified byincrementing the value of this last programmed page 608 by 1.Furthermore, in a case where data has been programmed to all the pages301 inside the block 202, the number of the last page is registered inthe last programmed page 608. Furthermore, data cannot be programmed tothis block 202 until erase processing has been performed.

The number of invalid pages 609 shows the number of invalid pages (pagesnot associated with a LBA 501) within the block 202 identified inaccordance with the block number 603.

Since it is not possible to update the memory chip 120 using anoverwrite, when the host computer 10 performs a data update with respectto an LBA 501, a new PBA 502 is associated with the LBA 501 (refer toFIG. 5). Then, data is programmed to the page 301 corresponding to thisnewly associated PBA 502. For this reason, a page 301 programmed withpre-update data becomes an invalid page, which loses its associationwith the LBA 501 and is not referenced from the host computer 10. Inthis example, the number of such invalid pages is managed for each block202, and a block 202 that is targeted for reclamation, which will beexplained further below, is selected based on the number of invalidpages.

The preceding has been an explanation of the block management table 600.Furthermore, in this example, the explanation gives an example in whichall the information shown in FIG. 6 is used, but the block managementtable 600 does not necessarily have to be made using all the types ofinformation shown in FIG. 6. Multiple control targets are possible incontrolling the command interval of the memory chip 120, such as “timebetween programming and erase”, “time between erase and programming”,“time between erase and next erase” and “time between programming andnext programming”, but the block management table 600 may be made usingonly a portion of the information columns shown in FIG. 6. For example,in a case where the control target is the “time between programming anderase”, the block management table 600 may be made using only the lastpage programming date/time 607. Furthermore, the size of the table 600will increase, but information denoting the programming date/time foreach page may be registered in the table 600. Then, the programmingdate/time for the PE interval or the EP interval does not necessarilyhave to be the programming date/time of either the first or the lastpage, but rather the date and time of programming of a page other thanthe first and last page may be used as the programming date/time for thePE interval or EP interval.

Furthermore, the various types of information of the block managementtable 600 (601 through 609) do not necessarily have to be recorded inthe RAM 113. For example, the PBA group 601, the chip number 602, theblock number 603, the number of erases 604, the previous erase date/time605, the first page programming date/time 606, and the last pageprogramming date/time 607 may be recorded with respect to the last pageof the block 202 identified in accordance with the block number 603included in this row for each row.

It is better that the period of time (PE interval) from the programmingto the erasing of data with respect to a single block 202 (one page 301)be long. In this example, when the FM controller 110 attempts to erasedata that has been programmed to a certain block 202, in a case wherethe time from the “last page programming date/time 607”, which is thetime at which data was programmed to the last page 301 of the pages 301that configure this erase process-target block 202, to the current timehas been detected and this time is equal to or longer than a certainthreshold (hereinafter, the PE threshold), the data programmed to theerase process-target block 202 is erased. In so doing, it is possible tocontrol the PE interval of all the pages 301 configuring the block 202so as to be equal to or larger than the PE threshold. This is becausethe time period from the programming of data to the page 301 prior tothe last page 301 until the present time will always be longer than thatof the last page 301. That is, the PE interval of the last page 301 inthe block 202 constitutes the shortest PE interval in this block 202,and the PE interval of the initial page 301 in this block 202constitutes the longest PE interval in this block 202.

It is desirable to manage the programming date/time for each page 301(or cell: a unit for configuring a page 301), but the managementinformation will become voluminous. Rather than managing, theprogramming date/time of all the pages configuring the block 202,managing the programming date/time for the last page and computing thePE interval based thereon as in this example makes it possible tocontrol the PE intervals for all the pages configuring the block 202 toequal to or larger than the PE threshold while preventing the size ofthe management information 1140 from becoming voluminous.

Furthermore, it is better that the period of time (EP interval) from theerasing to the programming of data with respect to a page 301 be short.In this example, in a case where the period of time from the “previouserase date/time 605” of the block 202 to which data is attempting to beprogrammed (programming-target block) has been detected and this time isequal to or less than a certain threshold (hereinafter, the EPthreshold), the data is programmed to the programming-target block 202.This makes it possible to control the EP interval to equal to or lessthan the EP threshold.

Furthermore, in a case where there is a block 202 for which the EPinterval (the period of time from the “previous erase date/time 605”until the current time) exceeds the EP threshold, the FM controller 110executes an erase process with respect to this block 202 even thoughdata has not been programmed to this block 202. In accordance with this,since the “previous erase date/time 605” is updated to the currentdate/time, the EP interval is reset to the initial value (for example,zero).

FIG. 7 shows an example of a wear leveling management queue 700.

The wear leveling management queue 700 is one of the pieces ofmanagement information stored in the RAM 114. The wear levelingmanagement queue 700 manages the number of erases of a block 202 (PBAgroup) by dividing the number of erases into levels of fixed numberintervals. The wear leveling management queue 700 associatively managesa range of erases 701 and a PBA group 702.

The range of erases 701 shows ranges of the number of erases (at leastone of the upper limit or lower limit of the number of erases). FIG. 7shows an example of five levels based on the statistics (for example,the average values) of the number of erases of all the blocks 202mounted in the FM module 100. That is, the range of erases will differsaccording to the average value of the number of erases of all the blocks202 mounted in the FM module 100. The range of erases may be updatedeither regularly or irregularly.

In FIG. 7, for example, the level with the fewest number of erases(range of erases) is the range in which the number of erases equal to orlarger than 201 is less than the average value, and thereafter, thenumber of erases is demarcated into a range of erases every 200 erases.In wear leveling, which equalizes the number of erases of a block, theequalization of the number of erases in realized by preferentially usingblocks from a block group with a small number of erases (bypreferentially using a block that belongs to the smallest levelpossible).

The PBA group 702 shows the first address of the block 202 that belongsto the range of erases 701.

The PBA group 702 is recorded as a FIFO (First In First Out) type queue,and a new PBA group 702 is registered at the end of the queue (the rightend in the example of FIG. 7). Then, when using the blocks, the block,which was registered the earliest chronologically, is used, and theblock registered at the end of the queue is not selected until all thepreviously registered blocks have been used.

Furthermore, in this example, an example in which the number of erasesshown in FIG. 7 is divided into five levels, i.e., one level every 200erases, was presented, but the present invention is not limited to theselevels. For example, the number of erases can be sorted into variousranges in accordance with the precision of the equalization of number oferases that is required. Furthermore, the range of erases does not needto be uniform. For example, one range of erases can be 200 erases, andanother range of erases can be 150.

The preceding has been an explanation of the management information usedin this example.

FIG. 8 shows an example of the programs stored in the RAM 114.

An I/O (Input/Output) program 1141, a reclamation program 1142, and anerase program 1143 are stored in the RAM 114. Various functions arerealized in accordance with these programs being executed by theprocessor 115. These programs will be explained in detail further belowusing flowcharts.

FIG. 9 shows an overview of erase process control.

The FM controller 110 checks the “number of invalid pages 609” of theblocks 202 configuring each memory chip 120 mounted in the FM module100, and detects a block for which the number of invalid pages 609 isgreater than a reference value (Step S1). The reference value may be aprescribed threshold, or may be the average value of the number ofinvalid pages in all the blocks 202. That is, in S1, a block with anabsolutely and/or relatively large number of invalid pages is detected.Here it is supposed that block 202A has been detected.

Next, the FM controller 110 computes the period of time from the “lastpage programming date/time 607” of the detected block 202A to thecurrent time, and checks whether this computed time period (elapsedtime) is equal to or larger than the PE threshold (S2). According to theexample shown in the drawing, the PE threshold is 3 h (h: hours). The“last page programming date/time 607” of the block 202A is“2010/4.14/9:00”. Since the current time is “2010/4.14/13:00”, theperiod of time until the present is 4 h. In this case, the FM controller110 determines that the period of time since the block 202A was lastprogrammed is equal to or larger than the PE threshold.

Therefore, the FM controller 110 executes an erase process with respectto the block 202A (S3).

Alternatively, even when block 202B is detected (S5), the FM controller110 does not carry out an erase process for block 202B. That is, sincethe “last page programming date/time 607” for block 202B is“2010/4.14/11:00” and the current time is “2010/4.14/13:00”, the periodof time until the present is 2 h. This period of time (2 h) is not equalto or larger than the PE threshold (3 h). For this reason, the FMcontroller 110 does not carry out an erase process with respect to theblock 202B (S6). In a case where the period of time from the “last pageprogramming date/time 607” to the present is equal to or larger than thePE threshold with respect to the block 202B, the FM controller 110 willexecute an erase process for the block 202B.

Thus, an erase process in not carried out for a block for which theperiod of time from the last page programming date/time 607 to thecurrent time is not equal to or larger than the PE threshold. For thisreason, the block PE interval is guaranteed to be the PE threshold.

FIG. 10 shows an overview of programming process control.

The FM controller 110 detects a post-erase process block (hereinafter,an erased block) 202C in which data is not written (S11). The FMcontroller 110 computes the period of time from the “previous erasedate/time 605” of this block 202C to the current time, and checkswhether or not the computed time period is equal to or less than the EPthreshold (S12).

In the example shown in the drawing, the EP threshold is 5 h. The“previous erase date/time 605” of the block 202C is “2010/4.15/9:00”.Since the current time is “2010/4.15/13:00”, the period of time up untilthe present (the elapsed time) is 4 h. In this case, the FM controller110 determines that the period of time from the “previous erasedate/time 605” of the block 202C is equal to or less than the EPthreshold. In this case, the FM controller 110 programs data to thefirst page of the block 202C (S13).

Alternatively, even when the FM controller 110 detects the erased block202D (S14), the FM controller 110 carries out an erase process withrespect to the block 202D without programming data to the first page ofthe block 202D. Specifically, the “previous first page programmingdate/time 606” of the block 202D is “2010/4.15/5:00”. Since the currenttime is “2010/4.15/13:00”, the period of time up until the present is 8h. In this case, the FM controller 110 determines that the period oftime (8 h) from the “previous erase date/time 605” of the block 202Dexceeds the EP threshold (S15). In this case, the FM controller 110carries out an erase process with respect to the block 202D withoutprogramming data to the first page to the block 202D (S16).

Thus, data is not written to the first page of an erased block for whichthe period of time from the “previous erase date/time 605” exceeds theEP threshold. Consequently, the creation of a block for which the EPinterval exceeds the EP threshold can be avoided.

The processing performed in this example will be explained in detailbelow.

FIG. 11 shows an example of the flow of a programming process. Theprocessor 115, for example, by executing the I/O program 1141, is ableto carry out the processing explained by referring to FIG. 11.

The FM module 100 receives a write command, which includes write-targetdata and an LBA specifying a write-target area, from the host computer10 (S1101). More specifically, the processor 115 is notified of a writecommand from the host computer 10 via the FE-IF 111. Next, the processor115 instructs the FE-IF 111 to write the write-target data to the databuffer 113. The FE-IF 111 programs the write-target data from the hostcomputer 10 to the data buffer 113.

Next, the processor 115 acquires the PBA associated with the LBAreceived in S1101 (S1102). This process is a characteristic process ofthis example, and as such will be explained in detail further below.

The processor 115 instructs the FM-IF 117 to create an ECC (ErrorCorrecting Code) (S1103). Next, the FM-IF 117 reads the write-targetdata from the data buffer 113 in units of ECC-protected data lengths,computes an ECC for each protected data length using the ECC creationcircuit inside the FM-IF 117, and programs this ECC and write-targetdata to the data buffer 113.

The processor 115 transfers the write-target data received in S1101 andthe ECC created in Step S1103 from the data buffer 113 to the memorychip 120, and carries out programming (S1104). Specifically, theprocessor 115 instructs the FM-IF 116 to transfer the write-target dataand the ECC created with respect thereto to the memory chip 120, andinstructs the FM-IF 116 to carry out programming. Next, upon receivingthe instructions, the FM-IF 116 reads the write-target data and the ECCfrom the data buffer 113, transfers this write-target data and ECC tothe memory chip 120, and instructs the memory chip 120 to performprogramming.. The memory chip 120 programs the data in accordance withthis instruction. The data programming destination is a pagecorresponding to the PBA identified in S1102.

In order to associate the LBA received in S1101 with the PBA acquired inS1102, the processor 115 updates the address conversion table (refer toFIG. 5) (S1105). By this processing, it becomes possible to acquire thePBA that is associated with the LBA from the host computer 10.Specifically, the processor 115 retrieves an LBA 501, which conforms tothe LBA specified in the write data, from the address conversion table500, and registers the PBA acquired in S1102 as the PBA 502corresponding to the discovered LBA 501.

The processor 115 updates the block management table 600 (S1106).Specifically, the following operations are performed.

(S1106-1) The processor 115 searches the block management table 600 forthe PBA group that conforms to the PBA acquired in S1102.

(S1106-2) Next, the processor 115 acquires the page number 301 of thepage, which is in the block (the target block in the explanation of FIG.11) of the PBA acquired in S1102, and which became the programmingdestination in S1104. The processor 115 registers this page number 301as the last page programmed 608.

(S1106-3) Furthermore, when this page number 301 is the first page ofthe target block 202, the processor 115 registers information denotingthe current time as the relevant first page programming date/time 606 ofthe block management table 600. When the page number 301 is the lastpage of the target block 202, the processor 115 registers informationdenoting the current time as the last page programming date/time 607.

The preceding has been an overview of the programming process.

Next, Step S1102, which is one of the characteristic features of thisexample, will be explained using FIG. 12.

When a long time has elapsed since an erase process was carried out, orwhen a long time has elapsed since data was programmed to a page, thereis a risk that the initial threshold voltage for the page 301 willfluctuate, making it impossible to apply a suitable charge to thefloating gate when programming and reducing the reliability of thestored data.

With respect to memory chip 120 characteristics like this, in thisexample, control is exercised to keep the elapsed time from the previouserase date/time and the elapsed time following programming a first pageto within a fixed period of time.

FIG. 12 shows an example of the flow of processing in Step S1102.

The processor 115 selects a candidate block from a group of blocks thathave been partially programmed (S1201).

Specifically, the processor 115 refers to the block management table 600and selects one arbitrary block from among multiple blocks 202 for whichthe last page programmed 608 does not denote the last page. As usedhere, a block 202 for which the last page programmed 608 does not denotethe last page is the “partially programmed block” referred to in theexplanation of FIG. 12, and the arbitrarily selected block is the“candidate block” referred to in the explanation of FIG. 12.

Furthermore, in S1201, the processor 115 may select a block for whichthe number of erases 604 is equal to or larger than a reference value (aprescribed threshold value, or a value obtained based on the number oferases of multiple blocks) as the partially programmed block that isable to become a candidate block. Since programming can bepreferentially carried out to a block 202 with a small number of erases604 in accordance with this, it is possible to lessen the drop in dataretention reliability brought on by the passage of time subsequent to anerase.

The processor 115 determines whether or not it is possible to select acandidate block (S1202). In a case where a candidate block is acquirable(S1202: Yes), the processor 115 identifies the first page programmingdate/time 606 of the selected candidate block (S1203).

The processor 115 computes the elapsed time from the first pageprogramming date/time 606 until the present (S1204).

The processor 115 determines whether or not the elapsed time computed inS1204 is equal to or less than a preconfigured threshold (hereinafter,the PP threshold) (S1205). In a case where the elapsed time is equal toor less than the PP threshold (S1205: Yes), the processor 115 identifiesthe PBA of the page subsequent to the last programmed page of thecandidate block (S1206). This is because reliability is maintained in acase where the elapsed time computed in S1204 is equal to or less thanthe PP threshold.

In a case where the elapsed time computed in S1204 exceeds the PPthreshold (S1205: No), the processor 115 removes the block selected inS1201 from the partially programmed block group (S1207). This is becauseof a risk that the drop in reliability will be outside an allowablerange in a case where the elapsed time computed in S1204 exceeds the PPthreshold. In S1207, specifically, for example, the processor 115 mayregister the value of the last page as the last page programmed 608 withrespect to the block selected in S1201, and may register informationdenoting the current time as the last page programming date/time 607.

Alternatively, in a case where it is not possible to select a candidateblock (typically, a case in which there are no partially programmedblocks) (S1202: No), the processor 115 refers to the wear levelingmanagement queue 700 and selects a block with a number of erases that isless than the reference value (S1208). The memory chip 120, as wasexplained hereinabove, comprises characteristics whereby reliabilitydecreases when the EP interval is long. For this reason, a controlmethod, which carries out an erase immediately prior to programming iseffective at enhancing reliability. However, in this example, based onthe characteristic that the time it takes to erase the memory chip 120is longer than the programming or read time, a block for which an eraseprocess has been carried out is prepared beforehand for the purpose ofenhancing the performance of the FM module 100. In this example,multiple erased blocks are divided into levels in accordance with thenumber of erases and registered in the wear leveling management queue700. In S1208, for example, a block with the lowest possible range oferases level is selected.

The processor 115 identifies the “previous erase date/time 605” of theblock selected in S1208.

The processor 115 computes the elapsed time from the previous erasedate/time 605 identified in S1209 until the current time (S1210).

The processor 115 checks whether or not the elapsed time computed inS1210 is equal to or less than the EP threshold (S1211). In a case wherethe elapsed time computed in S1210 is equal to or less than the EPthreshold (S1211: Yes), the processor 115 identifies the PBA of thefirst page of the block acquired in S1208 (S1213).

The processor 115, in a case where the elapsed time computed in S1210 isnot equal to or less than the EP threshold (S1211: No), executes anerase process for the block selected in S1208 (S1212). According to thisprocessing, the elapsed time from the previous erase date/time 605 untilthe current time is reset to the initial value (for example, zero). Theprocessor 115 identifies the PBA of the first page of the block forwhich the erase process was executed in S1212 (S1213).

According to S1211 and S1212, the block comprising the pagecorresponding to the PBA identified in S1213 is guaranteed to be a blockfor which the elapsed time from the previous erase date/time 605 untilto current time is equal to or less than the EP threshold.

In S1104 of FIG. 11, data is programmed to the page corresponding to thePBA identified in either S1206 or S1213.

The flow of a read process will be explained next.

FIG. 13 shows an example of the flow of read processing.

The FM controller 110 receives a read command specifying a LBA that willbe the read target (the target LBA in the explanation of FIG. 13) fromthe host computer 10 (S1301).

The processor 115 refers to the address conversion table 500, andidentifies the PBA allocated to the target LBA (S1302).

The processor 115 reads data from the page corresponding to the PBAidentified in S1302 (S1303). Specifically, the processor 115 issues aread command to the FE-IF 116. Upon receiving the command, the FM-IF 116specifies a block 202 and a page 301 to the memory chip 120 comprisingthe PBA-denoted page, and reads the data.

The FM-IF 116 uses the ECC appended to the data read in S1303 to checkthe integrity of the data that was read. When an error is detected inaccordance with this check, the FM-IF 116 revises the data that was readusing the ECC (S1304).

The FM-IF 116 transfers the read data to the host computer 10. In a casewhere the data was revised in S1304, the revised data is transferred tothe host computer 10 (S1305).

The preceding has been an explanation of read processing in thisexample.

Next, a reclamation process of this example will be explained. Inaddition to a request from the host computer 10, the reclamation processalso occurs collaterally in accordance with a write process.

First of all, the trigger for starting the reclamation process will beexplained.

As was explained hereinabove, the memory chip 120 possessescharacteristics whereby overwriting is not possible once data has beenprogrammed to a page. For this reason, when writes are carried out withrespect to the same LBA, data is programmed to the page subsequent tothe last page programmed, and the PBA of the page to which data has beenprogrammed anew is registered in the address conversion table 500 as thePBA corresponding to this LBA in place of the PBA of the page to whichthe pre-update data was programmed.

According to this control, a data update is carried out without the hostcomputer 10 being made aware of the fact that the PBA has changed.

When the above-mentioned update write is carried out, the programmablepages inside the FM module 100 decrease due to a new page being used foreach update write. For this reason, the FM module 100 of this examplecarries out a reclamation process in a case where the programmable pagesinside the FM module 100 have become equal to or less than a fixedquantity.

The reclamation process, for example, is carried out by the processor115 executing the reclamation program 1142. Specifically, the followingoperations are executed in the reclamation process.

-   (x1) The processor 115 searches for a block comprising invalid pages    equal to or greater than a reference value. As used here, the    “reference value” may be a prescribed threshold, or may be a value    based on the number of invalid pages in multiple blocks (for    example, an average value). That is, a block for which the number of    invalid pages is either absolutely or relatively large is searched    for here. The “invalid page” is a page for which a LBA has not been    allocated in the address conversion table 500 and to which data has    been programmed.-   (x2) Next, the processor copies the data inside a valid page (a page    allocated to an LBA in the address conversion table 500) included in    the block discovered in (x1)) to a page in another block, and    updates the association with the LBA of the address conversion table    500 to the copy-destination page. According to this processing, all    the pages inside the block discovered in (x1) are treated as invalid    pages. That is, this block is treated as an invalid block. An    invalid block is a block in which all the pages are invalid pages    (in other words, a block, which does not include a valid page    identified based on the PBA allocated to the LBA). The invalid block    may be subjected to an erase process.

In the reclamation process of this example, multiple invalid blocks(blocks, which are erase-target candidates) may be managed, and an eraseprocess with respect to the invalid block may be carried out in a casewhere the number of erased blocks is equal to or less than a threshold.Furthermore, this threshold may be decided in accordance with the writeperformance of the FM module 100.

Alternatively, according to memory chip 120 characteristics, when the PEinterval (the elapsed time from programming to an erase) is short, thenumber of electron traps and positive hole traps created in the oxideinsulation layer of a cell will be more numerous than when the PE timeinterval is long. For this reason, there is a risk that the reliabilityof a block with a short PE time interval will be low.

With respect to the above-mentioned characteristics, in this example, anerase process is not carried out for a block with a PE interval that isequal to or less than the PE threshold in erase processing triggered bya reclamation. As an exception, in a case where the number of erasedblocks has been exhausted, an erase process may be performed withrespect to a block for which the elapsed time from the last pageprogramming date/time 607 is equal to or less than the PE threshold tosecure an erased block, in so doing, the block, which undergoes theerase process, may be the block with the longest elapsed time from thelast page programming date/time 607 from among multiple blocks for whichthe elapsed time from the last page programming date/time 607 is equalto or less than the PE threshold. Furthermore, the erase process, forexample, may be carried out by the processor 115 executing the eraseprogram 1143.

FIG. 14 shows an example of the flow of reclamation processing.

The reclamation process is started when a prescribed event has beendetected, for example, when it has been detected that the number oferased blocks inside the FM module 100 is equal to or less than aprescribed threshold.

The processor 115 identifies from the block management table 600 a blockfor which the number of invalid pages 609 is either absolutely orrelatively large (S1401). The block selected in S1401 may be a block inwhich data has been programmed to the last page.

The processor 115 identifies the last page programming date: time 607 ofthe block identified in S1401 (the “candidate block” in the explanationof FIG. 14) (S1402).

The processor 115 obtains the elapsed time from the last pageprogramming date/time 607 for the candidate block by computing thedifference between the value of the last page programming date/time 607identified in S1402 and the current date/time (S1403).

The processor 115 checks whether or not the elapsed time computed inS1403 is less than the PE threshold (S1404). In a case where the elapsedtime computed in S1403 is less than the PE threshold (S1404: Yes), theprocessor 115 cancels the candidate block (S1405), and carries out S1401once again. In S1405, the processor 115 may write the number of thecanceled candidate block to the RAM 114, for example, so that thecanceled candidate block is not selected again. The reason whyprocessing like S1405 is carried out is because there is a risk of dataretention-related reliability dropping for the block for which theelapsed time obtained in S1403 is less than the PE threshold.

Alternatively, in a case where the elapsed time obtained in S1403 isequal to or larger than the PE threshold (S1404: No), the processor 115copies the data of all the valid pages inside the candidate block topages in another block (S1406). Then, the processor 115 associates thePBA of the copy-destination page with the LBA allocated to the PBA ofthe valid pages of the candidate block instead of the PBA thereof (anupdate of the address conversion table 500). According to thisprocessing, all the pages inside the candidate block become invalidpages. That is, the candidate block becomes an invalid block.Furthermore, in a case where the candidate block acquired in S1401 wasalready an invalid block, this step does not have to be executed, andthe processor 115 can execute the processing of S1407 subsequent to theprocessing of S1404.

The processor 115 carries out an erase process for the invalid blockcreated in S1406 (S1407).

The processor 115 adds 1 to the number of erases of the block for whichthe erase process was carried out in S1406. Then, the processor 115registers the erased block (candidate block) in the wear levelingmanagement queue 700 based on the post-addition number of erases(S1408). In accordance with this, the candidate block is associated withthe level of the range of erases to which the post-addition number oferases of the candidate block belongs.

The preceding has been an explanation of the reclamation process in thisexample. Furthermore, in this example, the elapsed time from the lastpage programming date/time 607 is compared to the PE threshold, but theelapsed time subjected to comparison is not limited thereto. Forexample, the elapsed time from the first page programming date/time 606until the current time may be compared to the PE threshold. Furthermore,the last page programming date/time 607 does not have to be a measuredvalue, and, for example, may be a value that was predicted from thefirst page programming date/time 606.

As has been described up to this point, in this example, an eraseprocess is carried out with respect to a block for which the elapsedtime from the last page programming date/time 607 is equal to or largerthan the PE threshold, but an erase process is not carried out for ablock for which this elapsed time is less than the PE threshold.Furthermore, data is programmed to the first page of a block for whichthe elapsed time from the previous erase date/time 605 is equal to orless than the EP threshold, but data is not programmed to the first pageof a block for which this elapsed time exceeds the EP threshold (data isprogrammed to the first page of this block after an erase process hasbeen carried out with respect to this block). Furthermore, in a casewhere the elapsed time from the first page programming date/time 606 isequal to or less than the PP threshold, data is programmed to the pagesubsequent to the last page that was programmed. In accordance with theabove-described control, it is possible to hold a drop in the dataretention-related reliability of the FM module 100 in check.Furthermore, since the fact that the PE interval will be equal to orlarger than the PE threshold is assured, the point at which the numberof erases reaches the upper limit can be delayed, thereby potentiallyextending the life of the FM module 100.

EXAMPLE 2

Example 2 will be explained below. In so doing, the explanation willfocus on the points of difference with Example 1, and explanations ofthose points in common with Example 1 will either be simplified oromitted (Furthermore, the same will hold true for Example 3 as well.)

In Example 1, a minimum time period (PE threshold) is stipulated for thePE interval, but in Example 2, instead of stipulating a PE threshold,control is implemented so as to ensure a PE interval to the extentpossible for individual blocks with respect to a command from the hostcomputer 10.

FIG. 15 shows an example of a PE interval management queue 1500.

The PE interval management queue 1500 is one of the pieces of managementinformation stored in the RAM 114. The PF, interval management queue1500 is management information, which is divided into groups inaccordance with the elapsed time from the last page programmingdate/time until the current date/time (hereinafter, will be called theelapsed programming time), and in which each group is registered in aPBA group of invalid blocks corresponding to a range of elapsed times.The PE interval management queue 1500 is configured from an elapsedprogramming time 1501 and a PBA group 1502.

The elapsed programming time 1501 is information denoting the elapsedprogramming time.

According to the example of FIG. 15, the elapsed programming time isdivided into four levels. The blocks are sorted into each of a number offixed ranges of elapsed programming times such that the block group thatbelongs to the level with the longest elapsed programming time is theblock group for which the elapsed programming time is 30 minutes orlonger, followed by the block group for which the elapsed programmingtime is more than 20 minutes but less than 29 minutes. In this example,the elapsed programming time is managed on the order of minutes and isnot managed on the order of seconds, but the unit of the elapsedprogramming time is not limited to minutes.

The PBA group 1502 stores the first address of a PBA group 1502 denotinga relevant block in each elapsed programming time level (range). The PBAgroup 1502 is registered as a FIFO type queue, and a new block isregistered at the end of the queue.

In Example 2, an invalid block is managed using the PE intervalmanagement queue 1500. When the number of invalid blocks becomes equalto or less than a fixed number, an invalid block is created using aninvalid block creation process (for example, the reclamation process),and the created invalid block is registered in the PE intervalmanagement queue 1500 based on the elapsed programming time of thisinvalid block. These operations will be explained in detail furtherbelow.

Furthermore, in Example 2, the PE interval management queue 1500 isupdated either irregularly or regularly, for example, every 10 minutes.Specifically, each time the PE interval management queue 1500 isupdated, the FM controller 110 measures the period of time from the timeat which this queue 1500 was updated until the present. In a case wherethe elapsed programming time of the invalid block at a point in time T(the point in time at which 10 minutes has elapsed since the measuredtime) belongs to a higher level than the level to which this invalidblock belonged at the point in time T, the FM controller 110 registersthe PBA of the first page of this invalid block at the end of the queueof this higher level.

Furthermore, in this example, the queue 1500 changes every 10 minutes,but the updating of the queue 1500 is not limited to 10 minuteintervals. For example, the queue 1500 may be updated in units of onesecond.

Next, the invalid block creation process in this example will beexplained.

FIG. 16 is an outline diagram showing a block utilization cycle inExample 2.

A data record block group 1601 is a block group conceptually showingblocks comprising valid pages. The FM controller 110 discerns the datarecord block group 1601 from the block management table 600.Furthermore, when the number of blocks registered in an invalid blockgroup 1602 is equal to or larger than a fixed number, the FM controller110 selects a block (that is, a block for which the invalid pages areeither absolutely or relatively numerous) with a number of invalid pagesthat is equal to or larger than a reference value from among the datarecord block group 1601. This selected block is a reclamation candidateblock. Then, the FM controller 110 copies all the valid pages of thereclamation candidate block to the pages of another block.

Next, the FM controller 110 changes the corresponding destination of theLBA to which the. valid pages of the reclamation candidate block havecorresponded, from the reclamation candidate block valid pages to thecopy-destination pages thereof. In accordance with this, all the validpages of the reclamation candidate block become invalid pages. As aresult of this, the reclamation candidate block becomes an invalidblock. An invalid block created in this way is registered in the invalidblock group 1602.

The invalid block group 1602 is a block group in which invalid blocks(blocks in which all the pages are invalid pages) have been registered.The FM controller 110 registers the invalid block in the PE intervalmanagement queue 1500 level to which the elapsed programming time ofthis invalid block corresponds. When the number of blocks configuringthe erased block group 1603 becomes equal to or less than a fixednumber, the FM controller 110 selects from among the invalid block group1602 a block for which the elapsed programming time is equal to orlarger than a reference value, and creates an erased block by carryingout an erase process with respect to the selected invalid block. Anerased block created in this way is registered in the erased block group1603. Furthermore, the reference value of the elapsed programming timemay be the prescribed PE threshold of the elapsed programming time, ormay be a value (for example, an average value) based on the elapsedprogramming times of the multiple invalid blocks configuring the invalidblock group 1602. That is, the selected invalid block is an invalidblock with either an absolutely or relatively long elapsed programmingtime.

The erased block group 1603 is a block group in which erased blocks havebeen registered. The FM controller 110 registers an erased block in thewear leveling management queue 700.

The FM controller 110, when a write command is issued from the hostcomputer 10 and partially programmed blocks (blocks with recordablepages in which data has not been recorded up to the last page) are equalto or less than a fixed number, selects from among the erased blockgroup 1603 a block for which the number of erases is equal to or lessthan a reference value, and programs write data received from the hostcomputer 10 to a page inside this selected block. In accordance withthis, this block becomes a data record block (a partially programmedblock). Furthermore, the reference value of the number of erases may bea prescribed threshold of the number of erases, or may be a value (forexample, an average value) based on the number of erases of the multipleerased blocks configuring the erased block group 1603. That is, theselected erased block is an erased block with an either absolutely orrelatively few number of erases.

The processing carried out by Example 2 will be explained in detailbelow.

FIG. 17 shows an example of the flow of invalid block creationprocessing. The processor 115, for example, can carry out the processingexplained using FIG. 17 by executing the invalid block creation program1144 (refer to FIG. 19).

The processor 115 selects an invalid block candidate (an invalidcandidate block) from the data record block group (S1701). Specifically,the processor 115 refers to the block management table 600 and selects ablock with an either absolutely or relatively large value stored in thenumber of invalid pages 609 as the invalid candidate block. Theprocessor 115 may select the block with the largest number of invalidpages from the table 600 as the invalid candidate block at this time.Furthermore, the criteria for the invalid block creation target is thethreshold of the number of invalid pages, and the processor 115 mayselect as the invalid candidate block a block for which the valueregistered in the number of invalid pages 609 is equal to or larger thanthe threshold.

The processor 115 identifies the last page programming date/time 607 forthe invalid candidate block selected in S1701 (S1702).

The processor 115 computes the elapsed time (elapsed programming time)from the last page programming date/time 607 identified in S1702 untilthe present (S1703).

The processor 115 copies all the valid pages inside the invalidcandidate block selected in S1701 to free pages (pages in which data isnot recorded, neither valid data nor invalid data) in another block, andchanges the corresponding destination of the LBA with which acopy-source valid page is associated from the copy-source valid page tothe copy-destination page thereof (S1704). in accordance with thisprocessing, all the pages inside the invalid candidate block becomeinvalid pages, and the invalid candidate block becomes an invalid block.

The processor 115 registers the invalid block (the candidate block inwhich all the pages became invalid pages) in the relevant level of theelapsed programming time computed in S1704 (S1705). For example, in acase where the elapsed programming time computed in S1703 is 12 minutes,the processor 115 records the PBA of the first page of the invalidcandidate block in the row of the PE interval management queue 1500 forwhich the PE interval is from 10 minutes to 19 minutes.

The preceding has been an explanation of the invalid block creationprocess of this example.

Next, the erase process of this example will be explained.

FIG. 18 shows an example of the flow of erase processing.

In the initial step of the erase process, the processor 115 refers tothe PE interval management queue 1500, and selects a block from thelevel with the longest elapsed programming time (S1801). In theexplanation of FIG. 18, the block selected in S1801 will be called the“erase candidate block”.

More specifically, the processor 115 refers to the PE intervalmanagement queue 1500, and identifies the first block (the block, whichwas registered the furthest in the past of all the blocks registered inthe group) of the longest elapsed programming time level (highest level)(30 minutes or more in the example of FIG. 15). In a case where a blockis not registered in the highest level at this time, the processor 115selects a block from the next highest level (from 20 minutes to 29minutes in the example of FIG. 15). In a case like this where a blockcannot be identified, the processor 115 drops down one level and selectsa block from this level. Thus, the scope of the block searchsequentially changes to a lower elapsed programming time level until ablock can be acquired.

The processor 115 refers to the block management table 600 andidentifies the value of the number of erases 604 for the rowcorresponding to the erase candidate block (S1802).

The processor 115 notifies the FM-IF 116 of the PBA denoting the erasecandidate block and an erase command, and the FM-IF 116 notifies thechip comprising the erase candidate block of the block number, and sendsthis chip the erase command (S1803).

The processor 115 increments by 1 the number of erases identified inS1802 (S1804). In accordance with this, a new number of erases isdecided for the erase candidate block.

The processor 115 uses the number of erases computed in S1804 toregister the candidate block (erased block), which implemented the eraseprocess in S1803, in the relevant level of the wear leveling managementqueue 700. The new number of erases is also registered in the number oferases 604 of the block management table 600 (S1805).

According to Example 2, an erase process is preferentially performedwith respect to an invalid block with a long elapsed programming time.In accordance with this, the PE intervals of all the blocks can be madelonger. As a result of this, the numbers of electron traps and positivehole traps generated when the PE interval is short are reduced, makingpossible higher reliability.

Furthermore, increasing reliability like this makes it possible toreduce blocks determined to be incapable of being used to store data,and to substantially increase the upper limit of the number of erases ofusable blocks, thereby enabling the life of the FM module 100 to beprolonged.

Furthermore, the present invention may be put into practice by combiningthe minimum time rule of the PE interval in Example 1 and the controlfor ensuring the PE interval to the extent possible in this example(that is, Example 1 and Example 2).

EXAMPLE 3

Up to this point, examples for restricting the minimum time of the PEinterval during normal processing (Example 1) or controlling the PEinterval for a long time (Example 2) have been described. These examplesmake it possible to curb the generation of numerous electron traps andpositive hole traps as a result of a shortened PE interval, and toprevent a drop in reliability related to the data retention of thenonvolatile semiconductor memory (data retention reliability).

Example 3 makes it possible to reduce the positive hole traps andelectron traps that occur and to enhance data retention reliability bydetecting a block having low data retention reliability and lengtheningthe PE interval for this block during operation. That is, this examplemakes it possible to reduce unusable blocks.

The data retention period in a nonvolatile semiconductor memory isgenerally limited. For this reason, it is preferable that processing becarried out to read and copy data to another location every fixedperiod. This processing is generally called a “refresh process”.Furthermore, the number of failed hits in a nonvolatile semiconductormemory will increase together with the passage of time following datarecording. For this reason, the FM controller 110, either regularly orirregularly, can read all the storage areas inside the FM module 100,and check the number of failed bits that have occurred on each page.This processing will be called a “verify process” in this example.

FIG. 20 shows an example of a table showing the relationship between anumber of elapsed days and a failed bit threshold.

When a number of days has passed since data was programmed, failed bitsincrease in the memory. For this reason, according to the table 2000shown in FIG. 20, when the value of a number of elapsed days 2001 (forexample, the number of days since programming data to a certain page(for example, either a first page or a last page) until the present) ishigh, a failed bit threshold 2002 is high. The failed bit threshold isthe threshold of the number of failed bits.

In this example, in at least one of a read process (the processing of aread command from the host computer 10), a refresh process, or a verifyprocess, the FM controller 110 identifies the failed bit threshold 2002corresponding to the number of elapsed days (for example, the number ofdays since programming data to a certain page (for example, either afirst page or a last page) until the present) with respect to a block.The FM controller 110 detects a block (hereinafter, will be called anat-risk block) for which the number of failed bits (or the averagenumber of failed bits of all the ECC CWs inside the block) is equal toor larger than a failed bit threshold. More specifically, the FMcontroller 110 first reads data from a page inside the block during anyof the above-mentioned processes. Then, the FM controller 110 acquiresthe number of failed bits for each ECC configuring this page. Next, theFM controller 110 identifies the block for which the number of failedbits is equal to or larger than the failed bit threshold as an at-riskblock. Furthermore, the failed bit threshold may be a value of equal toor less than the number of bits capable of being corrected using theECC.

A block, which has been determined to be an at-risk block, will becomethe target of the invalid block creation process described in Example 2.The FM controller 110 copies data of all the pages included in theat-risk block to another block, and updates the address conversion table500. In accordance with this processing, an at-risk block becomes aninvalid block. Then, the FM controller 110 registers this invalid blockin an at-risk block management queue (not shown in the drawing). Theat-risk block management queue is a FIFO type queue, and whenregistering a block in the queue, the block is registered at the end ofthe queue.

In Example 3, the invalid block is registered in the PE intervalmanagement queue 1500 the same as in Example 2.

FIG. 21 shows an example of a management screen 2100.

The management screen 2100 is a screen displayed on the managementcomputer 11. The management computer 11 manages multiple (or one) FMmodules 100.

The IDs (a list of modules) of multiple FM modules 100 managed by themanagement computer 11 are displayed in the management screen 2100. Ause can select one or more FM module IDs from the list of modules.

The management screen 2100 comprises a time display area 2101, and ablock display screen 2102 is displayed in front of this screen 2100.Information related to the FM module that was selected by the user fromthe multiple FM modules IDs is displayed in this screen 2102. Themanagement computer 11 may collect management information from themultiple FM modules 100, and either a portion or all of this managementinformation may be displayed in at least one of the screens 2100 and2102.

A “minimum allowable PE interval” and a “maximum allowable EP interval”are displayed in the time display area 2101. This “minimum allowable PEinterval” (that is, the PE threshold) and this “maximum allowable EPinterval” (that is, the EP threshold) cannot be changed by the user viathis management screen 2100 in this example, but the configuration maybe such that at least one of these thresholds can be changed by the uservia this screen 2100.

The block display screen 2102 displays a “total number of blocks” and a“number of blocks at risk” with respect to the FM module correspondingto the FM module ID selected by the user from the list of modules. The“total number of blocks” is the total number of blocks of the FM module100 selected by the user, and the “number of blocks at risk” is thenumber of at-risk blocks from among these blocks. The at-risk block, aswas described hereinabove, is a block for which the number of failedbits is equal to or larger than the failed bit threshold.

FIG. 22 shows an example of the flow of erase processing.

In this example, as was described hereinabove, a block with the numberof failed bits that is equal to or larger than the failed bit thresholdis managed as an at-risk block. The FM controller 110 as much aspossible makes sure that an at-risk block does not become the target ofan erase. This is because the memory chip 120 comprises characteristicswhereby, when left alone in a state in which data has been programmed,the number of failed bits occurring in data recorded at the time of thenext programming declines. The at-risk block is controlled so as tobecome an erase-target block after a prescribed period of time haspassed with respect to the elapsed programming time of this block.

The processor 115 determines whether or not the number of selectedblocks managed by the FM module 100 is less than a prescribed threshold(S2201). The threshold at this time, for example, is 100,000.

In a case where the number of selected blocks is less than the threshold(S2201: Yes), the same as in Example 2, the processor 115 selects aninvalid block from the PE interval management queue 1500 (S2202). Theprocessor 115 increments the number of selected blocks by 1. Therefore,the above-mentioned “number of selected blocks” denotes the number oftimes that an invalid block, which is not an at-risk block, has beenselected. The processing from S2204 through S2207 is substantially thesame as that of S1802 through S1805 (refer to FIG. 18) of Example 2, andas such, explanations will be omitted.

Alternatively, in a case where the number of selected blocks is equal toor larger than the prescribed threshold (S2201: No), the processor 115acquires a block (the erase-target block in the explanation of FIG. 22)from the at-risk block management queue (S2208). The fact that thenumber of selected blocks rose from zero to equal to or larger than thethreshold signifies that the at-risk block had been left alone and notselected for the period of time that it took the number of selectedblocks to increase from zero up to equal to or larger than thethreshold. Therefore, there is the likelihood that the number of failedbits occurring in data recorded at the time of the next programming tothe at-risk block will decline. The at-risk block is preferably a blockin which some sort of data has been programmed.

The processor 115 sets the number of selected blocks to the initialvalue (for example, 0) (S2209). In accordance with this, the at-riskblock will not be selected as an erase candidate block until the numberof selected blocks once again increases to equal to or larger than thethreshold.

Subsequent to S2209, the processor 115 carries out the processing ofS2204.

FIG. 23 shows an example of the flow of read processing.

The processing of S2301 through S2304 is substantially the same as thatof S1301 through S1304 of FIG. 13, and as such, explanations will beomitted.

The processor 115 identifies the number of failed bits of theread-source block (S2305).

The processor 115 identifies the failed bit threshold corresponding tothe number of elapsed programming days of the read-source block from thetable 2000 shown in FIG. 20, and checks whether or not the number offailed bits identified in S2305 is less than the identified failed bitthreshold (S2306).

In a case where the number of failed bits is less than the failed bitthreshold (S2306: Yes), the processor 115 transfers corrected data tothe host computer 10 (S1305).

Alternatively, in a case where the number of identified failed bits isequal to or larger than the failed bit threshold (S2306: No), theprocessor 115 registers the read-source block as an at-risk block(S2307).

According to Example 3, an at-risk block is selected as the erasecandidate block less frequently than an invalid block that is not anat-risk block. In accordance with this, the PE interval of the at-riskblock is extended. Extending the PE interval makes it possible detrapthe electron traps and positive hole traps that exist inside the oxidelayer of the at-risk block, to restore the data retention capabilitiesof the at-risk block, and to once again maintain data retentionreliability. In accordance with this, unusable blocks decrease, thenumber of erases that can be used increases, and the life of the FMmodule 100 can be prolonged.

Furthermore, according to the above explanation, the at-risk block is ablock with the number of failed bits that is equal to or larger than thefailed bit threshold, but the comparison of the number of failed bitsand the failed bit threshold may be carried out in prescribed units,such as, units of data, units of pages, or units of blocks.

Furthermore, either instead of or in addition to a block with the numberof failed bits that is equal to or larger than the field bit threshold,the at-risk block may be a block that corresponds to at least one of (1)and (2) below.

(1) A programming time that is less than a first reference value.

(*) A hole can be formed in the oxide insulation layer when a blockdegrades. A risk is that although N electrons were intended to be putinto the FG (floating gate), less than N electrons get into the FG andthe remainder of the electrons enter the hole in the oxide insulationlayer. In this case, the number of implanted electrons is less than N,causing the time required for programming to be shortened.

(*) The first reference value may be a prescribed threshold, or may be avalue (for example, an average value) based on the programming time (thetime required for programming) of multiple blocks. That is, this (1) maybe an “either absolutely or relatively long programming time”.

(2) A programming time that is less than a second reference value.

(*) When a block degrades, it is difficult to remove the electrons fromthe FG in an erase process.

(*) The second reference value may be a prescribed threshold, or may bea value (for example, an average value) based on the erase time (thetime required for an erase process) of multiple blocks. That is, this(2) may be an “either absolutely or relatively short erase time”.

A number of examples have been explained hereinabove, but the presentinvention is not limited to these examples.

For example, the LBA may be multi-tiered. For example, the LBA specifiedfrom the host computer 10 may be a first-tier LBA of an area in thelogical volume, a second-tier LBA recognized by the FM module 100 may beassociated with the first-tier LBA, and a PBA may be associated with thesecond-tier LBA. Even though the LBA is multi-tiered, the PBA isindirectly associated with the first-tier LBA.

Furthermore, for example, Example 3 includes Example 1 and Example 2,but Example 2 does not have to be included. In this case, for example,in S2202 of FIG. 22, a block with either absolutely or relativelynumerous invalid pages may be selected, and in S2203, the number ofselected blocks may be incremented by 1.

REFERENCE SIGNS LIST

-   100 FM module-   110 FM controller-   120 Memory (nonvolatile semiconductor memory)

1-15. (canceled)
 16. A flash memory module comprising: a plurality offlash memory chips including a plurality of blocks, wherein each blockof the plurality of blocks is a unit of erasing data; and a flashcontroller coupled to the plurality of flash memory chips, wherein theflash controller is configured to program data to a block and erase datafrom a block, and wherein the flash controller is configured to: manage,for each block of the plurality of blocks, a recent programming timethat is a time at which data is recently programmed to a block; anderase data stored in a block for which an elapsed programming time islarger than a first value, wherein the elapsed programming time is atime period from the recent programming time.
 17. The flash memory ofclaim 16, wherein each block comprises multiple pages, and is a type ofblock for which the erase process is carried in units of blocks andprogramming is carried out in units of pages, in relation to controllinga timing at which an erase process is carried out with respect to ablock based on the recent programming time of the block, the recentprogramming time of the block is a time at which data is recentlyprogrammed to a last page of this block, and the target block is aninvalid block that does not comprise a valid page, which is a pageidentified from a physical address allocated to a logical address. 18.The flash memory of claim 17, wherein the first value is a value basedon the elapsed programming times of the multiple blocks.
 19. The flashmemory of claim 18, wherein the controller detects an at-risk block,wherein the at-risk block is a data-programmed block for which there isa risk that reliability drops, and in controlling a timing at which anerase process is carried out with respect to a block based on the recentprogramming time of this block, the controller preferentially selects aninvalid block instead of the at-risk block from the multiple blocks asthe target block.
 20. The flash memory of claim 19, wherein, incontrolling a timing at which an erase process is carried out withrespect to a block based on the recent programming time of this block,the controller is configured to: on a condition that a number ofselected blocks is less than a second value, select an invalid block asthe target block and update the number of selected blocks, wherein thenumber of selected blocks is the number of times that invalid blockshave been selected; and on a condition that the number of selectedblocks is equal to or larger than the second value, select the at-riskblock as the target block, and return the number of selected blocks toan initial value.
 21. The flash memory of claim 20, wherein the at-riskblock is a block for which the number of failed bits is equal to orlarger than a failed bit threshold, and the controller identifies, fromamong multiple failed bit thresholds, a failed bit thresholdcorresponding to the elapsed programming time of the block, and in acase where the number of failed bits of this block is equal to or largerthan this identified failed bit threshold, detects that this block is anat-risk block.
 22. The flash memory of claim 21, wherein the elapsedprogramming time includes a first time period, which is a time periodfrom the time at which data is recently programmed to the last page, anda second time period, which is a time period from the time at which datais recently programmed to a page other than the last page, and theelapsed programming time related to the controller detecting an at-riskblock is the second time period.
 23. A flash memory module comprising: aplurality of flash memory chips including a plurality of blocks, whereineach block of the plurality of blocks is a unit of erasing data; and aflash controller coupled to the plurality of flash memory chips, whereinthe flash controller is configured to program data to a block and erasedata from a block, and wherein the flash controller is configured to:manage, for each of the plurality of blocks, a recent erasing time thatis a time at which data is recently erased from a block; and programdata to a block for which an elapsed erasing time is less than a firstvalue, wherein the elapsed erasing time is a time period from the recenterasing time.
 24. The flash memory module of claim 23, wherein, incontrolling a timing at which data is programmed to a block based on atleast one of the recent programming time and the recent erase time ofthe block, the controller carries out the erase process with respect toa block for which the elapsed erase time is equal to or larger than thethird value, thereby making this block the target block.
 25. The flashmemory module of claim 23, wherein each block comprises multiple pages,and is a type of block for which the erase process is carried in unitsof blocks and programming is carried out in units of pages, and inrelation to the controlling a timing at which data is programmed to ablock based on at least one of the recent programming time and therecent erase time of the block, the recent programming time of a blockis a time at which data is recently programmed to a first page of thisblock.
 26. The flash memory module of claim 23, wherein, in thecontrolling a timing at which data is programmed to a block based on atleast one of the recent programming time and the recent erase time ofthe block, the controller is configured to: on a condition that nopartially programmed block exists, wherein a partially programmed blockis a block in which data is programmed at least to a first page but datais not programmed to a last page, select the erased block as the targetblock; and on a condition that the partially programmed block exists,when a time period from the time at which the partially programmed blockis recently programmed is less than a fourth value, program data to apage subsequent to a rearmost programmed page.
 27. The flash memorymodule of claim 11, wherein, in the controlling a timing at which anerase process is carried out with respect to a block based on the recentprogramming time of the block, the controller carries out the eraseprocess with respect to a block for which an elapsed programming time,which is a time period from the recent programming time, is equal to orlarger than a first value, thereby making this block the erased block.